Existing compensation structures for Electric Power Steering (EPS) systems often use torque loop compensation for an electric motor control system without motor velocity compensation. As shown in FIG. 1, in such an existing EPS system employing voltage mode control, a torque compensator 101 commonly a notch filter is employed in the torque path to provide phase lead to ensure that the system remains stable. Since the motor (46 FIG. 2) is run in voltage (pseudo current) mode, back electromotive force (BEMF) compensation proportional to the measured motor speed is added to the voltage command.
Such an EPS system is disclosed and described in commonly assigned U.S. Pat. No. 5,719,766 to Bolourchi et al. While well suited for its intended purposes, this system may be sensitive to motor velocity disturbances and there is no effective way of rejecting them in present structure because the compensation is in the torque path. The torque compensator 101 depicted in the torque path is used for stability and the high pass gain 104 of the high frequency path is used for torque disturbance rejection. Application of a high frequency path to the torque loop compensation of the EPS system is disclosed and described in commonly assigned U.S. Pat. No. 5,704,446 to Chandy et al. Application of a torque compensator 101 makes the EPS system sensitive to disturbances that include frequency content near the notch frequency. Lower notch frequencies result in greater sensitivity of the control system to the torque disturbances near the notch frequency.
With the aforementioned considerations, it has been difficult to apply a control architecture such as that depicted in FIG. 1 to some vehicles and tune the control system to achieve acceptable performance. Excessive high-pass gains 104 may be needed to improve the disturbance rejection at lower frequencies, which unfortunately, increases high frequency disturbance sensitivity. In fact, there may be instances where high-frequency disturbances may not be completely rejected with any stable tuning employed.